Popular types of built-in type microphones to be mounted on a disk-top system personal computer or a mobile system personal computer (hereinafter referred to as a personal computer) include a non-directional condenser microphone and a boundary microphone.
A condenser microphone is a non-directional microphone which is not much affected by a breath of a speaker or wind. Therefore, the microphone can be made relatively flat with respect to a flat face on which a keyboard or a display panel is provided, and fit in a flat face of housing for a thin shaped personal computer. However, since the condenser microphone is the non-directional microphone, it is designated to provide a small difference between the surrounding noise and the objective sound. Therefore, there is a tendency for such the microphone to have difficulties in catching excellent sound and imparting the personal computer to normally recognize voice sound.
Conversely, the boundary microphone is parallel to a fat-faced surface of the housing on which the keyboard or the display panel is provided, when it is mounted on the personal computer. Therefore, the length of that direct sound transmits from the outside into a front acoustic terminal, would be generally same as the length of that a reflected sound by the mounted surface transmits into the front acoustic terminal. Thereby, it is possible to prevent the microphone from a loss in clarity of sound quality caused by their different phases.
However, in the case of the boundary microphone, in order to input a clearer sound from a voice of a speaker, a main axis of the microphone has to be directed toward the speaker. Particularly, since a personal computer was often set on a desk or on a speaker's legs, the keyboard surface or the display panel surface of the computer with the built-in boundary microphone would not always be directed toward the speaker. Therefore, for the personal computer which clearer input voice is required, it is not reasonable to incorporate the built-in boundary microphone in the personal computer.
It can be considered that the unidirectional microphone capable of detecting only the front voice would be built in the personal computer in place of the non-directional microphone. However, a size of the unidirectional microphone has to be considered. Because the unidirectional microphone has a shape of an elongate or an oblong construction, since the design enables a sound input from both front and back sides of a diaphragm of the unidirectional microphone.
When the unidirectional microphone is fit in a flat-faced housing surface of the personal computer, not only stylish design but also compact design for the microphone are required. Therefore, the unidirectional microphone should not be increased in a thickness of main body thereof, when it is mounted on the personal computer.
On the other hand, it is considered that a recess would be provided on the housing surface of the personal computer with the keyboard or a display panel for fitting an unidirectional microphone therein. However, in such the way, the adequate pressure gradient would not be obtained, since the unidirectional microphone did not have different sound pressures between the front acoustic terminal and a rear acoustic terminal. Therefore, there is a problem that an unidirectional performance of the unidirectional microphone falls to a performance near the non-directional microphone, despite of unidirectivity.